Generally, a filter such as electromagnetic interference (EMI) filter is provided in power converters for filtering and suppressing electromagnetic interference in the converters.
The filter is an electronic circuit apparatus for suppressing electromagnetic interference in a power circuit or a control signal circuit. The function of the filter is to keep noise generated within electronic equipment from leaking out, and at the same time to block noise generated in an AC circuit outside of the electronic equipment from entering into the equipment. Usually, the filter contains passive electronic components including capacitors and inductors, wherein the capacitors usually include X-capacitors, i.e., a safety capacitor for eliminating differential mode interference. The X-capacitor is typically positioned between input lines of an AC power supply, for example between Line L and Line N. During operation, the X-capacitor is charged and thus stored unsafe levels of high-voltage energy, therefore the X-capacitor should be discharged rapidly after the AC power supply is disconnected so as to avoid accidents.
FIG. 1 is an X-capacitor discharge circuit in the prior art. In the prior art, for security, an energy consumption resistor is provided as a discharge circuit and is connected in parallel with an X-capacitor Cx. As shown in FIG. 1, the energy consumption resistor includes two consumption resistors R1 and R2. After the AC power supply is disconnected, the energy consumption resistor and the X-capacitor Cx constitute a loop to discharge the stored energy to a safe level within a short time. The energy consumption resistor is referred to as a discharge resistor of the X-capacitor Cx.
However, when the power converter is in a normal operation state, namely, an AC power is supplied to the power converter, a power consumption of Vac2/(R1+R2) is contributed by the energy consumption resistor of the X-capacitor, wherein Vac (Voltage alternating current, AC voltage) denotes the AC voltage. For example, when Vac=230 v (volt) and R1+R2=2MΩ (megohm), P=Vac2/(R1+R2)=26.45 mw (milliwatt), i.e., the energy consumption resistor produces about 26 mw power loss. The larger capacitance of the X-capacitor Cx is, the less of the required resistance values of the energy consumption resistors R1 and R2 are, and then the more of the power are consumed.
From above, the discharge circuit in the power converter in the prior art is always a contributor to the power consumption, especially when a larger capacitor is needed in consideration of suppressing electromagnetic interferences, a larger amount of power will be consumed by the discharge resistor. Meanwhile, with the development of technique, the requirement to the consumable energy in light load condition, which is negatively effected by the power consumption of the discharge resistor of the capacitor, is more and more rigorous.